1. Field of the Invention
The present invention refers to a gripper adapted to most effectively, and in a semi-automatic manner, seize, ie. hold firmly, and release cylindrical bodies.
Although, for reasons of greater descriptive simplicity, reference will be made to preforms of thermoplastic material used in the production of containers in the following description, the present invention shall of course to be understood as applying also to other cylindrical bodies used in different applications, as far as they do not depart from the scope of the appended claims.
2. Description of Related Art
It is known, from DE 29713510 and FR 2720679, grippers for seizing and firmly holding preforms comprise a handling arm, a pair of symmetrical, elastically diverging members, and connecting means between said arm and said symmetrical members. The latter are shaped so as to form a concavity whose opposite arms are arranged and sized in such a manner as to enable a cylindrical portion of the collar of a preform to be inserted therein by means of a simple pressure of said collar against the outer portions of said concavity, which, owing to the action of such a pressure, are caused to be enlarged by such an extent as adequate to enable the collar of the preform to slip into said concavity in which it is then retained.
However, such an operation and the particular type of gripper used to carry it out, drawbacks as described below.
The preform may be subjected to both axial and transversal stresses. In order to be able to withstand such stresses., the symmetrical members must possess a considerable elastic strength.
However, the required extent of the elastic strength brings about a considerable friction between the arms and the outer surface of the preform, which therefore undergoes a slight abrasion, considering that it is made of thermoplastic material that is only slightly heated up. In a situation of large-scale series production. The slight abrasions cause tiny particles, ie. the so-called xe2x80x9cflourxe2x80x9d of the plastic material involved to be produced, which contaminate the adjacent parts of the plant and disturb the regular flow of the subsequent process operations thereby making them inaccurate.
If, on the other hand, such an undesired effect has to be reduced, the need arises for the elastic gripping force of the arms to be reduced. Such a constraint, however, is largely known to all those skilled in the art to bring about corresponding inaccuracies in the positioning of the preform, with clear handling difficulties resulting therefrom.
Moreover, making grippers provided with holding arms having a considerable elastic strength implies a production that requires a far higher extent of accuracy and, therefore, is much more expensive.
It is therefore a main purpose of the present invention to provide a gripper for handling cylindrical bodies, and in particular plastic preforms, which does away with the above described drawbacks and, at the same time, is compact, safe and reliable in its operation, and requires a simple, reliable construction based on the use of readily available techniques.
Such a type of gripper is obtained and implemented with the characteristics that are substantially described with particular reference to the appended claims.